In the Global System for Mobile Telecommunications/General Packet Radio Service (GSM/GPRS) network architecture, as shown in FIG. 13, there are known data protocol stacks associated the various architectural elements, including the mobile station (MS), base station subsystem (BSS) including Base Transceiver Station (BTS) and Base Station Controller (BSC), serving GPRS support node (SGSN) and gateway GPRS support node (GGSN). The MS and the SGSN share peer logical link control (LLC) and subnetwork-dependent convergence protocol (SNDCP) layers in the user plane.
A typical GPRS negotiation that is required between peer entities in the mobile station and some of the fixed network devices is the exchange identification or XID negotiation, where so-called L3CE (layer 3 compatibility entity) parameters are agreed upon.
The UMTS packet network architecture is highly similar to GPRS. However, the naming of some elements and interfaces has been changed from GPRS. While FIG. 13 shows the GPRS network architecture, FIG. 14 shows the UMTS packet network architecture.
The UMTS packet network consists of the following network elements:    Node B: corresponds to Base Transceiver Station (BTS) in GSM.    RNC (Radio Network Controller): corresponds to Base Station Controller (BSC) in GSM.    3G-SGSN: the 3rd Generation version of the Serving GPRS Support Node (SGSN) of GSM/GPRS.    3G-GGSN: the 3rd Generation version of the Gateway GPRS Support Node (GGSN).    HLR: the GSM Home Location Register (HLR) with some updates.
As shown in FIG. 14, Node B and RNC comprise the RAN part of the UMTS network. RAN corresponds to GSM's BSS. The responsibility of RAN is the handling of all radio specific functions, e.g., radio channel ciphering, power control, radio bearer connection setup and release. The basic separation between elements is that Node B handles the physical layer functions and RNC handles the management functions. However, the separation might ultimately turn out to be slightly different than in GSM/GPRS.
The biggest architectural difference is the new interface, Iur, inside RAN. It is resident between RNCs. UMTS introduces a new concept called macrodiversity. In a macrodiversity situation, data is sent via multiple Node Bs. Because signals are transferred via multiple routes over the air interface and combined in the MS and the RNC, e.g., the fading effect is less harmful and thus lower power levels can be used. However, those Node Bs may belong to the area of two or more different RNCs, so the interface, i.e., Iur-interface between RNCs is required. In this situation, as shown on the right in FIG. 15, RNC can be in two logical roles. RNC can be logically either:                drift RNC (DRNC) or        serving RNC (SRNC).        
The actual termination point of the Iu-interface is at the SRNC. The Iu-interface shown in FIG. 14 connects the Radio Access Network (RAN) and Core Network (CN) for packet-switched or circuit switched services. The SRNC controls information transfer and requests radio resources from appropriate DRNCs. The DRNC only relays information between MS and SRNC.
The Core Network (CN) part of the packet-switched side consists of 3G-SGSN, 3G-GGSN and HLR elements, as shown in FIG. 14. The Packet Core Network (CN) also includes the IP-based backbone network. The backbone connects core network elements, e.g., 3G-SGSN and 3G-GGSN together.
3G-SGSN participates in routing of user packets as well as mobility and session management functions. The Mobility Management (MM) layer knows “who you are (security) and where you are (mobility)”. The Session Management (SM) layer controls the user connections, i.e., sessions.
3G-GGSN maintains the location information of 3G-SGSN, which serves the mobile station to which a packet is targeted. The main function of 3G-GGSN is to perform interworking functions between the UMTS network and the external data network, e.g., the Internet. These interworking functions include, e.g., the mapping of the external QoS to a comparable UMTS QoS.
HLR stores the subscriber data and holds the information to which 3G-SGSN the user is connected. The subscriber data includes predefined QoS attributes for the user connections, among other things.
The UMTS packet data protocol stack has some major modifications compared to GPRS, partly due to the new radio interface technology (WCDMA) and partly due to much higher QoS requirements.
One of the most important changes is that Logical Link Control layer (LLC) of ESM/GPRS has been removed below the Layer 3 Compatibility Entity (L3CE). L3CE corresponds to SubNetwork Dependent Convergence Protocol (SNDCP) protocol in GPRS. The main tasks of the LLC protocol have been:                flow control between MS and core network,        ciphering,        signaling message transfer,        multiplexing of different QoS and        retransmission between MS and the core network.        
In UMTS, LLC is not needed due to the following reasons: 1) Ciphering has been decided to take place in lower layers, inside RAN. 2) Signaling message transfer does not use user plane protocols, because there are separate protocols for transferring signaling messages and thus the differentation between the user plane and the control plane is clearer than in GPRS.
In the UMTS radio interface, each radio bearer will have its own Radio Link Control (RLC) entity. By applying this approach the QoS provisioning is more efficient. The QoS related multiplexing will be a task for the Medium Access Control (MAC) layer and Layer 1 (L1) and thus LLC would not have any role in QoS multiplexing in UMTS. The retransmission between the MS and the core network cannot be easily justified. The main source of the errors is the radio interface, and RLC has the responsibility to correct those errors.
However, the removal of LLC will cause a lack of flow control between the MS and the core network. The flow control in the uplink is not a problem, because the radio interface will be the bottleneck and flow control of RLC takes care of it. In the downlink, RLC will handle the RNC—MS part. Between RNC and the core network, there is no flow control. But this is not a much worse situation than in GPRS, because GPRS does not have any flow control inside the core network (between GGSN and SGSN).
Adequate data transfer between 3G-GGSN and RNC relies on large enough buffers, traffic policing in 3G-GGSN and end-to-end flow control, e.g., Transmission Control Protocol (TCP). In general, the removing of LLC streamlines the protocol stack and makes it easier to achieve higher data rates and reduces required processing power.
The location of the UMTS counterpart to L3CE (SNDCP in GPRS) called Packet Data Convergence Protocol (PDCP) is under consideration. Unlike in GPRS the PDCP layer is located in RNC instead of SGSN. The protocol inter alia takes care of optimization, e.g., by header compression, which is a form of optimization algorithm. Some header compression algorithms are based on the principle that disappearance of a few packets may cause undesirable additional packet loss due to the algorithm itself. This degrades packet transfer because more retransmissions are needed to be done. By locating it to the RNC, the retransmission time is short and the TCP level retransmission (due to TCP timers) can be avoided.
Network layer protocols are intended to be capable of operating over services derived from a wide variety of subnetworks and data links. The PDCP supports several network layer protocols providing protocol transparency for the users of the service. An introduction of new network layer protocols to be transferred over PDCP should be possible without any changes to other UMTS protocols. Therefore, all functions related to transferring of Network Layer Protocol Data Units (N-PDUs) are carried out in a transparent way by the network entities. Another requirement for PDCP is to provide functions that improve data and channel efficiency. This is done by different kind of optimization algorithms or methods, e.g., the above-mentioned header compression.
UMTS (Universal Mobile Telecommunications System), as shown in FIG. 14, utilizes similar protocol structures and negotiation arrangements for communication between mobile stations, Radio Network Controllers (RNCs) and service nodes of packet-switched networks, with some modification. Exchange Identification (XID) negotiation is carried out by the PDCP but is called PDCP parameter negotiation and can be viewed generally as a transfer of optimization algorithm parameters.
In either case, the negotiated parameters will relate to such optimization algorithm parameters, for example, to the use of headers and data compression. The GSM/GPRS method for arranging an XID negotiation is to insert the proposed parameters into certain messages in an LLC protocol layer and to use corresponding LLC-level answering messages to either acknowledge or reject the proposed SNDCP parameters.
The XID negotiation is usually made when SNDCP and LLC in GPRS are initialized (values for XID parameters are no longer valid). This initialization is made, e.g., when the MS is powered on or the location of network side protocols changes in handover.
The main problem of the currently-proposed XID negotiation method for UMTS is that the location of PDCP is different from the location of SNDCP and LLC protocol. PDCP locates in the radio access network while comparable GPRS protocols locate in core networks. This means that the location of PDCP changes far more often than the locations of SNDCP and LLC. Because XID messages may be relatively large, this adds much more overhead to the air interface in UMTS than in GPRS.
Another problem is that UMTS has also real time packet connections. This means that negotiations such as XID should be as fast as possible, because otherwise it may cause delays or at least more overhead in the air interface (header compression cannot be used after handover until XID negotiation is successfully made).